The miniaturization processes of analytical microsystem instruments for chemical and biological assay are well known. For many microsystem applications, glass is the preferred material over metal, silicon, and some plastics, because of its transparency and intertness to biological contamination and to many caustic chemicals. However a preferred material, silicate glass, is not used often because of the difficulty of microfabricating embedded fluid channels and the difficulty of sealing open trench surface channels. Typically, fabrication of a channel or a tunnel requires cutting a trench out of a workpiece and sealing this trench with another piece of glass that is aligned and bonded on top of the open trench. A processing step could be eliminated and system reliability improved if tunnels and other three dimensional cavities could be embedded within a monolithic piece of glass. It is desirable to reliably form three dimensional embedded glass structures using suitable batch fabrication processing.
Microelectromechanical Systems (MEMS) typically disadvantageously uses two dimensional processing steps to form three dimensional structures in bulk materials. Much of the two dimensional processing is based on silicon processing steps. However, semiconductors are not always optimal materials for use in some MEMS applications. One alternative class of materials is photostructurable glass ceramics, also known as photocerams. These photostructurable materials are typically exposed by using ultraviolet lamps and patterns are created using shadowmasks. The glass is exposed to a critical dose and the exposed material can subsequently be converted to a crystalline phase during a heating step. Prior processes utilize lamps that have a spectral emission that overlaps the absorption region or absorption band of the photostructurable material. In this absorption region, the workpiece responds to light exposure in a way that the critical dose is constant and the product of irradience and exposure time. This would be considered a linear exposure process. Photostructurable glass ceramics are a promising class of materials for MEMS devices. Previous micromachining work with these materials used conventional two dimensional photolithography equipment and masking techniques. Microdrilling techniques have been tried for both regular glass and photoceramic glass without much success. Standard two dimensional photolithographic techniques can only form planar two dimensional structures used in MEMS fabrication processing to describe extruded shapes, features formed by projecting a two dimensional pattern on a two dimensional surface of a workpiece. Current processes can not be used to create potentially useful three dimensional photoceramic structures such as a simple buried tunnel in a monolithic material. Furthermore, photolithographic processing of a monolithic material is not amenable to undercutting of patterned structures in a way that leaves anchored or suspended structures.
One prior method selectively patterns a Foturan material workpiece. Foturan is a lithium aluminosilicate glass with trace amounts of silver, cerium and antimony. Foturan is a trademark of Schott Glass. The Foturan material is a photostructurable glass ceramic available in wafer form. Standard Foturan processing involves exposure of an ultraviolet (UV) lamp through a prepatterned shadowmask. Foturan has a strong spectral absorption band beginning at wavelengths lesser than 350 nm. Hence, lamps with a spectral emission characteristic of 270 nm to 330 nm are used to expose the Foturan material because the emission band of the lamps falls within the strong absorption band of the Foturan material. Regions of the Foturan that accumulate a sufficient dose of UV light can then be converted to a crystalline material by heating the sample and etching the exposed material. Increasing the dose is achieved by increasing the intensity of the UV lamp across the spectral emission band of the lamp and or by increasing the irradiation time. The trade between intensity and irradiation time to achieve a critical dose is inverse and first order. The cerium dopant is believed to be a photosensitizer. In the presence of ultraviolet light, Ce3+ loses an electron to become Ce4+. A fraction of the free electrons reduce trapped Ag+ atoms to Ag0. In a subsequent thermal processing during heating, silver atoms diffuse together to form clusters. If a cluster is larger than 80 Å, the cluster can provide the nucleus for the growth of crystalline phase material into an amorphous phase material. The crystalline phase material is composed largely of lithium metasilicate that is preferentially soluble in hydrofluoric (HF) acid. Soaking the sample in an HF solution results in dissolution of the exposed regions leaving unexposed patterned structures. The crystallized material is highly soluble in the HF acid so that soaking the patterned and heated workpiece in HF causes the converted regions to dissolve away leaving the unexposed glass. However, the UV exposure is commonly directed to fabricating structures showing surface relief and is unsuitable for dissolving the crystalline material embedded within the workpiece. Three dimensional structure fabrication in photocerams can be achieved using the standard method with a lamp. However, embedded structures are difficult to obtain because lamp spectra is too broad and weak for effective exposure within the workpiece. The photons in part of the UV spectral region of the lamp are readily absorbed in the material and this inundates the volume above the desired embedded volume during patterning formation. The strong absorption results in the conversion of all regions above the embedded volume to the soluble phase. Attempts to use an unfiltered lamp would convert the embedded trench structure to a trench structure open to the surface. Focusing the lamp with broad spectrum light is also unsuitable for forming a stacked embedded structure because regions above and below the volume at the depth of focus may also accumulate a dose, and over subsequent exposure operations would convert to the crystalline phase preventing the formation of precise three dimensional embedded structures. These and other disadvantages are solved or reduced using the invention.